I. Field of the Invention
This invention relates to steering systems for articulated vehicles, and more particularly to a steering system which combines vehicle articulation with steerable movement of one axle of the vehicle.
II. Description of Related Art Including Information Disclosed Under Sections 1.97 and 1.98
It is highly desirable to provide a steering system for off-road type vehicles such as tractors, graders, or scrapers, to name a few which provides a small turning radius and which also provides readily responsive steering control on rough terrain. An articulated vehicle having four wheel drive has been found to provide a small turning radius, while an even smaller turning radius can be obtained if one of the axles of the articulated vehicle is steerable, typically called wagon-wheel type steering. Articulation of the vehicle is typically provided by mounting a pair of hydraulic cylinders between the two portions of the vehicle which are pivotally secured to each other. The cylinders are mounted on opposite sides of the pivot point, such that causing the extension of a first cylinder and the retraction of the second cylinder will cause the vehicle to turn in a first direction, while retraction of the first cylinder and extension of the second cylinder will cause the vehicle to turn in the opposite direction. The steerable axle, usually the front axle is provided by mounting a hydraulic cylinder between a pivotally mounted axle and the portion of the vehicle on which it is pivoted. To effectively control the steering of a vehicle having both articulation and steerable axle steering, it is desirable that a system be provided which effectively monitors the position and rate of change of position of the articulated body portions with respect to each other, the position and rate of change of position of the steerable axle with respect to the body portion on which it is mounted, and the position and rate of change of position of the steering control or controls manipulated by the vehicle operator, and utilizes inputs from the monitored conditions to control actuation of the hydraulic cylinders in response to an operator's input to provide a combination and timing of steering most responsive to the operator's input.
U.S. Pat. No. 4,771,851 entitled: Steering Control System for Articulated Vehicle, granted on Sep. 20, 1988 to Nystueun et al, assigned to the assignee of the subject application, sets forth the three position and rate of change of position inputs mentioned above, and discloses a steering control system which utilizes the three position and two of the rate of change of position inputs set forth above. The teaching of U.S. Pat. No. 4,771,851 are incorporated herein by reference.